Tristetraprolin is required for full anti-inflammatory response of murine macrophages to IL-10

IL-10: the master immunomodulatory cytokine in allergen immunotherapy

INTRODUCTION

Allergen immunotherapy (AIT) is the only disease-modifying treatment for patients with IgE-mediated allergic diseases. Successful AIT can induce long-term immune tolerance to the common allergen, which provides clinical benefits for years after discontinuation. The cytokine interleukin (IL)-10, as a key anti-inflammatory mediator with strong immunoregulatory functions, has drawn increasing attention over the past decades.

AREAS COVERED

After an extensive search of PubMed, EMBASE, and Web of Science databases, covering articles published from 1989 to 2024, our review aims to emphasize the key common information from previous reviews on the crucial involvement of IL-10 in allergen immunotherapy (AIT) induced immunological tolerance. In this review, we discuss the regulation of IL-10 expression and the molecular pathways associated with IL-10 function. We also further summarize mechanisms of immune tolerance induced by AIT, especially the indispensable role of IL-10 in AIT.

EXPERT OPINION

IL-10 plays an indispensable role in immune tolerance induced by AIT. Understanding the importance of the role of IL-10 in AIT would help us comprehend the mechanisms thoroughly and develop targeted therapeutics for allergic diseases.

IL-10 dependent adaptation allows macrophages to adjust inflammatory responses to TLR4 stimulation history

During an infection, innate immune cells must adjust nature and strength of their responses to changing pathogen abundances. To determine how stimulation of the pathogen sensing TLR4 shapes subsequent macrophage responses, we systematically varied priming and restimulation concentrations of its ligand KLA. We find that different priming strengths have very distinct effects at multiple stages of the signaling response, including receptor internalization, MAPK activation, cytokine and chemokine production, and nuclear translocation and chromatin association of NFκB and IκB members. In particular, restimulation-induced TNF-α production required KLA doses equal to or greater than those used for prior exposure, indicating that macrophages can detect and adaptively respond to changing TLR4 stimuli. Interestingly, while such adaptation was dependent on the anti-inflammatory cytokine IL-10, exogenous concentrations of IL-10 corresponding to those secreted after strong priming did not exert suppressive effects on TNF-α without such prior priming, confirming the critical role of TLR4 stimulation history.

RNA-binding proteins in vascular inflammation and atherosclerosis

Atherosclerotic cardiovascular disease (ASCVD) remains the major cause of premature death and disability worldwide, even when patients with an established manifestation of atherosclerotic heart disease are optimally treated according to the clinical guidelines. Apart from the epigenetic control of transcription of the genetic information to messenger RNAs (mRNAs), gene expression is tightly controlled at the post-transcriptional level before the initiation of translation. Although mRNAs are traditionally perceived as the messenger molecules that bring genetic information from the nuclear DNA to the cytoplasmic ribosomes for protein synthesis, emerging evidence suggests that processes controlling RNA metabolism, driven by RNA-binding proteins (RBPs), affect cellular function in health and disease. Over the recent years, vascular endothelial cell, smooth muscle cell and immune cell RBPs have emerged as key co- or post-transcriptional regulators of several genes related to vascular inflammation and atherosclerosis. In this review, we provide an overview of cell-specific function of RNA-binding proteins involved in all stages of ASCVD and how this knowledge may be used for the development of novel precision medicine therapeutics.

HUWE1 controls tristetraprolin proteasomal degradation by regulating its phosphorylation

Tristetraprolin (TTP) is a critical negative immune regulator. It binds AU-rich elements in the untranslated-regions of many mRNAs encoding pro-inflammatory mediators, thereby accelerating their decay. A key but poorly understood mechanism of TTP regulation is its timely proteolytic removal: TTP is degraded by the proteasome through yet unidentified phosphorylation-controlled drivers. In this study, we set out to identify factors controlling TTP stability. Cellular assays showed that TTP is strongly lysine-ubiquitinated, which is required for its turnover. A genetic screen identified the ubiquitin E3 ligase HUWE1 as a strong regulator of TTP proteasomal degradation, which we found to control TTP stability indirectly by regulating its phosphorylation. Pharmacological assessment of multiple kinases revealed that HUWE1-regulated TTP phosphorylation and stability was independent of the previously characterized effects of MAPK-mediated S52/S178 phosphorylation. HUWE1 function was dependent on phosphatase and E3 ligase binding sites identified in the TTP C-terminus. Our findings indicate that while phosphorylation of S52/S178 is critical for TTP stabilization at earlier times after pro-inflammatory stimulation, phosphorylation of the TTP C-terminus controls its stability at later stages.

Conceptual Advances in Control of Inflammation by the RNA-Binding Protein Tristetraprolin

Regulated changes in mRNA stability are critical drivers of gene expression adaptations to immunological cues. mRNA stability is controlled mainly by RNA-binding proteins (RBPs) which can directly cleave mRNA but more often act as adaptors for the recruitment of the RNA-degradation machinery. One of the most prominent RBPs with regulatory roles in the immune system is tristetraprolin (TTP). TTP targets mainly inflammation-associated mRNAs for degradation and is indispensable for the resolution of inflammation as well as the maintenance of immune homeostasis. Recent advances in the transcriptome-wide knowledge of mRNA expression and decay rates together with TTP binding sites in the target mRNAs revealed important limitations in our understanding of molecular mechanisms of TTP action. Such orthogonal analyses lead to the discovery that TTP binding destabilizes some bound mRNAs but not others in the same cell. Moreover, comparisons of various immune cells indicated that an mRNA can be destabilized by TTP in one cell type while it remains stable in a different cell linage despite the presence of TTP. The action of TTP extends from mRNA destabilization to inhibition of translation in a subset of targets. This article will discuss these unexpected context-dependent functions and their implications for the regulation of immune responses. Attention will be also payed to new insights into the role of TTP in physiology and tissue homeostasis.

Molecular Pathogenesis and Immune Evasion of Vesicular Stomatitis New Jersey Virus Inferred from Genes Expression Changes in Infected Porcine Macrophages

The molecular mechanisms associated with the pathogenesis of vesicular stomatitis virus (VSV) in livestock remain poorly understood. Several studies have highlighted the relevant role of macrophages in controlling the systemic dissemination of VSV during infection in different animal models, including mice, cattle, and pigs. To gain more insight into the molecular mechanisms used by VSV to impair the immune response in macrophages, we used microarrays to determine the transcriptomic changes produced by VSV infection in primary cultures of porcine macrophages. The results indicated that VSV infection induced the massive expression of multiple anorexic, pyrogenic, proinflammatory, and immunosuppressive genes. Overall, the interferon (IFN) response appeared to be suppressed, leading to the absence of stimulation of interferon-stimulated genes (ISG). Interestingly, VSV infection promoted the expression of several genes known to downregulate the expression of IFNβ. This represents an alternate mechanism for VSV control of the IFN response, beyond the recognized mechanisms mediated by the matrix protein. Although there was no significant differential gene expression in macrophages infected with a highly virulent epidemic strain compared to a less virulent endemic strain, the endemic strain consistently induced higher expression of all upregulated cytokines and chemokines. Collectively, this study provides novel insights into VSV molecular pathogenesis and immune evasion that warrant further investigation.

Post-Transcriptional Regulation of Immune Responses and Inflammatory Diseases by RNA-Binding ZFP36 Family Proteins

Post-transcriptional regulation is involved in the regulation of many inflammatory genes. Zinc finger protein 36 (ZFP36) family proteins are RNA-binding proteins involved in messenger RNA (mRNA) metabolism pathways. The ZFP36 family is composed of ZFP36 (also known as tristetraprolin, TTP), ZFP36L1, ZFP36L2, and ZFP36L3 (only in rodents). The ZFP36 family proteins contain two tandemly repeated CCCH-type zinc-finger motifs, bind to adenine uridine-rich elements in the 3’-untranslated regions (3’ UTR) of specific mRNA, and lead to target mRNA decay. Although the ZFP36 family members are structurally similar, they are known to play distinct functions and regulate different target mRNAs, probably due to their cell-type-specific expression patterns. For instance, ZFP36 has been well-known to function as an anti-inflammatory modulator in murine models of systemic inflammatory diseases by down-regulating the production of various pro-inflammatory cytokines, including TNF-α. Meanwhile, ZFP36L1 is required for the maintenance of the marginal-zone B cell compartment. Recently, we found that ZFP36L2 reduces the expression of Ikzf2 (encoding HELIOS) and suppresses regulatory T cell function. This review summarizes the current understanding of the post-transcriptional regulation of immunological responses and inflammatory diseases by RNA-binding ZFP36 family proteins.

Deficiency of Tristetraprolin Triggers Hyperthermia through Enhancing Hypothalamic Inflammation

Tristetraprolin (TTP), an RNA-binding protein, controls the stability of RNA by capturing AU-rich elements on their target genes. It has recently been identified that TTP serves as an anti-inflammatory protein by guiding the unstable mRNAs of pro-inflammatory proteins in multiple cells. However, it has not yet been investigated whether TTP affects the inflammatory responses in the hypothalamus. Since hypothalamic inflammation is tightly coupled to the disturbance of energy homeostasis, we designed the current study to investigate whether TTP regulates hypothalamic inflammation and thereby affects energy metabolism by utilizing TTP-deficient mice. We observed that deficiency of TTP led to enhanced hypothalamic inflammation via stimulation of a variety of pro-inflammatory genes. In addition, microglial activation occurred in the hypothalamus, which was accompanied by an enhanced inflammatory response. In line with these molecular and cellular observations, we finally confirmed that deficiency of TTP results in elevated core body temperature and energy expenditure. Taken together, our findings unmask novel roles of hypothalamic TTP on energy metabolism, which is linked to inflammatory responses in hypothalamic microglial cells.

Macrophage reprogramming for therapy

Dysfunction of the immune system underlies a plethora of human diseases, requiring the development of immunomodulatory therapeutic intervention. To date, most strategies employed have been focusing on the modification of T lymphocytes, and although remarkable improvement has been obtained, results often fall short of the intended outcome. Recent cutting-edge technologies have highlighted macrophages as potential targets for disease control. Macrophages play central roles in development, homeostasis and host defence, and their dysfunction and dysregulation have been implicated in the onset and pathogenesis of multiple disorders including cancer, neurodegeneration, autoimmunity and metabolic diseases. Recent advancements have led to a greater understanding of macrophage origin, diversity and function, in both health and disease. Over the last few years, a variety of strategies targeting macrophages have been developed and these open new therapeutic opportunities. Here, we review the progress in macrophage reprogramming in various disorders and discuss the potential implications and challenges for macrophage-targeted approaches in human disease.

Role of Tristetraprolin in the Resolution of Inflammation

Simple Summary

Chronic inflammatory diseases account for up to 60% of deaths worldwide and, thus, are considered a great threat for human health by the World Health Organization. Nevertheless, acute inflammatory reactions are an integral part of the host defense against invading pathogens or injuries. To avoid excessive damage due to the persistence of a highly reactive environment, inflammations need to resolve in a coordinate and timely manner, ensuring for the immunological normalization of the affected tissues. Since post-transcriptional regulatory mechanisms are essential for effective resolution, the present review discusses the key role of the RNA-binding and post-transcriptional regulatory protein tristetraprolin in establishing resolution of inflammation.

Abstract

Inflammation is a crucial part of immune responses towards invading pathogens or tissue damage. While inflammatory reactions are aimed at removing the triggering stimulus, it is important that these processes are terminated in a coordinate manner to prevent excessive tissue damage due to the highly reactive inflammatory environment. Initiation of inflammatory responses was proposed to be regulated predominantly at a transcriptional level, whereas post-transcriptional modes of regulation appear to be crucial for resolution of inflammation. The RNA-binding protein tristetraprolin (TTP) interacts with AU-rich elements in the 3′ untranslated region of mRNAs, recruits deadenylase complexes and thereby facilitates degradation of its targets. As TTP regulates the mRNA stability of numerous inflammatory mediators, it was put forward as a crucial post-transcriptional regulator of inflammation. Here, we summarize the current understanding of the function of TTP with a specific focus on its role in adding to resolution of inflammation.

HuD regulates SOD1 expression during oxidative stress in differentiated neuroblastoma cells and sporadic ALS motor cortex

The neuronal RNA-binding protein (RBP) HuD plays an important role in brain development, synaptic plasticity and neurodegenerative diseases such as Parkinson's (PD) and Alzheimer's (AD). Bioinformatics analysis of the human SOD1 mRNA 3' untranslated region (3'UTR) demonstrated the presence of HuD binding adenine-uridine (AU)-rich instability-conferring elements (AREs). Using differentiated SH-SY5Y cells along with brain tissues from sporadic amyotrophic lateral sclerosis (sALS) patients, we assessed HuD-dependent regulation of SOD1 mRNA. In vitro binding and mRNA decay assays demonstrate that HuD specifically binds to SOD1 ARE motifs promoting mRNA stabilization. In SH-SY5Y cells, overexpression of full-length HuD increased SOD1 mRNA and protein levels while a dominant negative form of the RBP downregulated its expression. HuD regulation of SOD1 mRNA was also found to be oxidative stress (OS)-dependent, as shown by the increased HuD binding and upregulation of this mRNA after H₂O₂ exposure. This treatment also induced a shift in alternative polyadenylation (APA) site usage in SOD1 3'UTR, increasing the levels of a long variant bearing HuD binding sites. The requirement of HuD for SOD1 upregulation during oxidative damage was validated using a specific siRNA that downregulated HuD protein levels to 36% and prevented upregulation of SOD1 and 91 additional genes. In the motor cortex from sALS patients, we found increases in SOD1 and HuD mRNAs and proteins, accompanied by greater HuD binding to this mRNA as confirmed by RNA-immunoprecipitation (RIP) assays. Altogether, our results suggest a role of HuD in the post-transcriptional regulation of SOD1 expression during ALS pathogenesis.

Cross-talk between CD38 and TTP Is Essential for Resolution of Inflammation during Microbial Sepsis

The resolution phase of acute inflammation is essential for tissue homeostasis, yet the underlying mechanisms remain unclear. We demonstrate that resolution of inflammation involves interactions between CD38 and tristetraprolin (TTP). During the onset of acute inflammation, CD38 levels are increased, leading to the production of Ca²⁺-signaling messengers, nicotinic acid adenine dinucleotide phosphate (NAADP), ADP ribose (ADPR), and cyclic ADPR (cADPR) from NAD(P)⁺. To initiate the onset of resolution, TTP expression is increased by the second messengers, NAADP and cADPR, which downregulate CD38 expression. The activation of TTP by Sirt1-dependent deacetylation, in response to increased NAD⁺ levels, suppresses the acute inflammatory response and decreases Rheb expression, inhibits mTORC1, and induces autophagolysosomes for bacterial clearance. TTP may represent a mechanistic target of anti-inflammatory agents, such as carbon monoxide. TTP mediates crosstalk between acute inflammation and autophagic clearance of bacteria from damaged tissue in the resolution of inflammation during sepsis.

Targeting Janus Kinases and Signal Transducer and Activator of Transcription 3 to Treat Inflammation, Fibrosis, and Cancer: Rationale, Progress, and Caution

Before it was molecularly cloned in 1994, acute-phase response factor or signal transducer and activator of transcription (STAT)3 was the focus of intense research into understanding the mammalian response to injury, particularly the acute-phase response. Although known to be essential for liver production of acute-phase reactant proteins, many of which augment innate immune responses, molecular cloning of acute-phase response factor or STAT3 and the research this enabled helped establish the central function of Janus kinase (JAK) family members in cytokine signaling and identified a multitude of cytokines and peptide hormones, beyond interleukin-6 and its family members, that activate JAKs and STAT3, as well as numerous new programs that their activation drives. Many, like the acute-phase response, are adaptive, whereas several are maladaptive and lead to chronic inflammation and adverse consequences, such as cachexia, fibrosis, organ dysfunction, and cancer. Molecular cloning of STAT3 also enabled the identification of other noncanonical roles for STAT3 in normal physiology, including its contribution to the function of the electron transport chain and oxidative phosphorylation, its basal and stress-related adaptive functions in mitochondria, its function as a scaffold in inflammation-enhanced platelet activation, and its contributions to endothelial permeability and calcium efflux from endoplasmic reticulum. In this review, we will summarize the molecular and cellular biology of JAK/STAT3 signaling and its functions under basal and stress conditions, which are adaptive, and then review maladaptive JAK/STAT3 signaling in animals and humans that lead to disease, as well as recent attempts to modulate them to treat these diseases. In addition, we will discuss how consideration of the noncanonical and stress-related functions of STAT3 cannot be ignored in efforts to target the canonical functions of STAT3, if the goal is to develop drugs that are not only effective but safe. SIGNIFICANCE STATEMENT: Key biological functions of Janus kinase (JAK)/signal transducer and activator of transcription (STAT)3 signaling can be delineated into two broad categories: those essential for normal cell and organ development and those activated in response to stress that are adaptive. Persistent or dysregulated JAK/STAT3 signaling, however, is maladaptive and contributes to many diseases, including diseases characterized by chronic inflammation and fibrosis, and cancer. A comprehensive understanding of JAK/STAT3 signaling in normal development, and in adaptive and maladaptive responses to stress, is essential for the continued development of safe and effective therapies that target this signaling pathway.

Biology and therapeutic potential of interleukin-10

The authors review the molecular mechanisms regulating IL-10 production and response and describe classic and novel functions of IL-10 in immune and non-immune cells. They further discuss the therapeutic potential of IL-10 in different diseases and the outstanding questions underlying an effective application of IL-10 in clinical settings.

The cytokine IL-10 is a key anti-inflammatory mediator ensuring protection of a host from over-exuberant responses to pathogens and microbiota, while playing important roles in other settings as sterile wound healing, autoimmunity, cancer, and homeostasis. Here we discuss our current understanding of the regulation of IL-10 production and of the molecular pathways associated with IL-10 responses. In addition to IL-10’s classic inhibitory effects on myeloid cells, we also describe the nonclassic roles attributed to this pleiotropic cytokine, including how IL-10 regulates basic processes of neural and adipose cells and how it promotes CD8 T cell activation, as well as epithelial repair. We further discuss its therapeutic potential in the context of different diseases and the outstanding questions that may help develop an effective application of IL-10 in diverse clinical settings.

Context-Dependent IL-1 mRNA-Destabilization by TTP Prevents Dysregulation of Immune Homeostasis Under Steady State Conditions

The bioavailability of the major pro-inflammatory cytokines IL-1α and IL-1β is tightly controlled by transcription and post-translational processing to prevent hyperinflammation. The role of mRNA decay in maintenance of physiological IL-1 amounts remained unknown. Here we show that the down-regulation of Il1a and Il1b mRNA by the mRNA-destabilizing protein TTP (gene Zfp36) is required for immune homeostasis. The TTP deficiency syndrome, a multi organ inflammation in TTP-/- mice, was significantly ameliorated upon deletion of the IL-1 receptor. Il1a and Il1b played non-redundant roles in triggering the pathological IL-1 signaling in TTP-/- mice. Accordingly, tissues from TTP-/- animals contained increased amounts of Il1b mRNA. Unexpectedly, TTP destabilized Il1b mRNA in cell type-specific ways as evident from RNA-Seq and mRNA stability assays. These results demonstrate that TTP-driven mRNA destabilization depends on the cellular context. Moreover, such context-defined mRNA decay is essential for keeping steady state IL-1 levels in the physiological range.

Interleukin-10 control of pre-miR155 maturation involves CELF2

The anti-inflammatory cytokine interleukin-10 (IL10) is essential for attenuating inflammatory responses, which includes reducing the expression of pro-inflammatory microRNA-155 (miR155) in lipopolysaccharide (LPS) activated macrophages. miR155 enhances the expression of pro-inflammatory cytokines such as TNFα and suppresses expression of anti-inflammatory molecules such as SHIP1 and SOCS1. We previously found that IL10 interfered with the maturation of pre-miR155 to miR155. To understand the mechanism by which IL10 interferes with pre-miR155 maturation we isolated proteins that associate with pre-miR155 in response to IL10 in macrophages. We identified CELF2, a member of the CUGBP, ELAV-Like Family (CELF) family of RNA binding proteins, as protein whose association with pre-miR155 increased in IL10 treated cells. CRISPR-Cas9 mediated knockdown of CELF2 impaired IL10’s ability to inhibit both miR155 expression and TNFα expression.

Through Reducing ROS Production, IL-10 Suppresses Caspase-1-Dependent IL-1β Maturation, thereby Preventing Chronic Neuroinflammation and Neurodegeneration

Chronic neuroinflammation contributes to the pathogenesis of Parkinson’s disease (PD). However, cellular and molecular mechanisms by which chronic neuroinflammation is formed and maintained remain elusive. This study aimed to explore detailed mechanisms by which anti-inflammatory cytokine interleukin-10 (IL-10) prevented chronic neuroinflammation and neurodegeneration. At 24 h after an intranigral injection of lipopolysaccharide (LPS), levels of NLRP3, pro-caspase-1, pro-IL-1β, active caspase-1, and mature IL-1β in the midbrain were much higher in IL-10^−/− mice than wildtype mice. Mechanistically, IL-10^−/− microglia produced more intracellular reactive oxygen species (iROS) and showed more profound activation of NADPH oxidase (NOX2) than wildtype microglia. Meanwhile, suppression of NOX2-derived iROS production blocked LPS-elicited caspase-1 activation and IL-1β maturation in IL-10^−/− microglia in vitro and in vivo. One month after intranigral LPS injection, IL-10^−/− mice revealed more profound microglial activation and dopaminergic neurodegeneration in the substantia nigra than wildtype mice. Importantly, such PD-like pathological changes were prevented by IL-1β neutralization. Collectively, IL-10 inhibited LPS-elicited production of NOX2-derived iROS thereby suppressing synthesis of NLRP3, pro-caspase-1 and pro-IL-1β and their activation and cleavage. By this mechanism, IL-10 prevented chronic neuroinflammation and neurodegeneration. This study suggested boosting anti-inflammatory effects of IL-10 and suppressing NLRP3 inflammasome activation could be beneficial for PD treatment.

Cooperative and distinct functions of MK2 and MK3 in the regulation of the macrophage transcriptional response to lipopolysaccharide

The p38^MAPK downstream targets MAPKAP kinases (MK) 2 and 3 are critical for the regulation of the macrophage response to LPS. The extents to which these two kinases act cooperatively and distinctly in regulating LPS-induced inflammatory cytokine expression are still unclear. To address this uncertainty, whole transcriptome analyses were performed using bone marrow-derived macrophages (BMDM) generated from MK2^−/− or MK2/3^−/− animals and their wild-type littermates. The results suggest that in BMDM, MK2 and MK3 not only cooperatively regulate the transcript expression of signaling intermediates, including IL-10, IL-19, CXCL2 and the IL-4 receptor (IL-4R)α subunit, they also exert distinct regulatory effects on the expression of specific transcripts. Based on the differential regulation of gene expression by MK2 and MK3, at least six regulatory patterns were identified. Importantly, we confirmed our previous finding, which showed that in the absence of MK2, MK3 negatively regulates IFN-β. Moreover, this genome-wide analysis identified the regulation of Cr1A, NOD1 and Serpina3f as similar to that of IFN-β. In the absence of MK2, MK3 also delayed the nuclear translocation of NFκB by delaying the ubiquitination and subsequent degradation of IκBβ, reflecting the substantial plasticity of the response of BMDM to LPS.

Dual-Specificity Phosphatases in Immunity and Infection: An Update

Kinase activation and phosphorylation cascades are key to initiate immune cell activation in response to recognition of antigen and sensing of microbial danger. However, for balanced and controlled immune responses, the intensity and duration of phospho-signaling has to be regulated. The dual-specificity phosphatase (DUSP) gene family has many members that are differentially expressed in resting and activated immune cells. Here, we review the progress made in the field of DUSP gene function in regulation of the immune system during the last decade. Studies in knockout mice have confirmed the essential functions of several DUSP-MAPK phosphatases (DUSP-MKP) in controlling inflammatory and anti-microbial immune responses and support the concept that individual DUSP-MKP shape and determine the outcome of innate immune responses due to context-dependent expression and selective inhibition of different mitogen-activated protein kinases (MAPK). In addition to the canonical DUSP-MKP, several small-size atypical DUSP proteins regulate immune cells and are therefore also reviewed here. Unexpected and complex findings in DUSP knockout mice pose new questions regarding cell type-specific and redundant functions. Another emerging question concerns the interaction of DUSP-MKP with non-MAPK binding partners and substrate proteins. Finally, the pharmacological targeting of DUSPs is desirable to modulate immune and inflammatory responses.

EP4 Antagonist-Elicited Extracellular Vesicles from Mesenchymal Stem Cells Rescue Cognition/Learning Deficiencies by Restoring Brain Cellular Functions

Adult brains have limited regenerative capacity. Consequently, both brain damage and neurodegenerative diseases often cause functional impairment for patients. Mesenchymal stem cells (MSCs), one type of adult stem cells, can be isolated from various adult tissues. MSCs have been used in clinical trials to treat human diseases and the therapeutic potentials of the MSC‐derived secretome and extracellular vesicles (EVs) have been under investigation. We found that blocking the prostaglandin E₂/prostaglandin E₂ receptor 4 (PGE₂/EP₄) signaling pathway in MSCs with EP₄ antagonists increased EV release and promoted the sorting of specific proteins, including anti‐inflammatory cytokines and factors that modify astrocyte function, blood–brain barrier integrity, and microglial migration into the damaged hippocampus, into the EVs. Systemic administration of EP₄ antagonist‐elicited MSC EVs repaired deficiencies of cognition, learning and memory, inhibited reactive astrogliosis, attenuated extensive inflammation, reduced microglial infiltration into the damaged hippocampus, and increased blood–brain barrier integrity when administered to mice following hippocampal damage. stem cells translational medicine2019

Triggering the Resolution of Immune Mediated Inflammatory Diseases: Can Targeting Leukocyte Migration Be the Answer?

Leukocyte recruitment is a pivotal process in the regulation and resolution of an inflammatory episode. It is vital for the protective responses to microbial infection and tissue damage, but is the unwanted reaction contributing to pathology in many immune mediated inflammatory diseases (IMIDs). Indeed, it is now recognized that patients with IMIDs have defects in at least one, if not multiple, check-points regulating the entry and exit of leukocytes from the inflamed site. In this review, we will explore our understanding of the imbalance in recruitment that permits the accumulation and persistence of leukocytes in IMIDs. We will highlight old and novel pharmacological tools targeting these processes in an attempt to trigger resolution of the inflammatory response. In this context, we will focus on cytokines, chemokines, known pro-resolving lipid mediators and potential novel lipids (e.g., sphingosine-1-phosphate), along with the actions of glucocorticoids mediated by 11-beta hydroxysteroid dehydrogenase 1 and 2.

RNA-Binding Proteins in the Control of LPS-Induced Macrophage Response

Innate immune response is triggered by pathogen components, like lipopolysaccharides (LPS) of gram-negative bacteria. LPS initiates Toll-like receptor 4 (TLR4) signaling, which involves mitogen activated protein kinases (MAPK) and nuclear factor kappa B (NFκB) in different pathway branches and ultimately induces inflammatory cytokine and chemokine expression, macrophage migration and phagocytosis. Timely gene transcription and post-transcriptional control of gene expression confer the adequate synthesis of signaling molecules. As trans-acting factors RNA binding proteins (RBPs) contribute significantly to the surveillance of gene expression. RBPs are involved in the regulation of mRNA processing, localization, stability and translation. Thereby they enable rapid cellular responses to inflammatory mediators and facilitate a coordinated systemic immune response. Specific RBP binding to conserved sequence motifs in their target mRNAs is mediated by RNA binding domains, like Zink-finger domains, RNA recognition motifs (RRM), and hnRNP K homology domains (KH), often arranged in modular arrays. In this review, we focus on RBPs Tristetraprolin (TTP), human antigen R (HUR), T-cell intracellular antigen 1 related protein (TIAR), and heterogeneous ribonuclear protein K (hnRNP K) in LPS induced macrophages as primary responding immune cells. We discuss recent experiments employing RNA immunoprecipitation and microarray analysis (RIP-Chip) and newly developed individual-nucleotide resolution crosslinking and immunoprecipitation (iCLIP), photoactivatable ribonucleoside-enhanced crosslinking (PAR-iCLIP) and RNA sequencing techniques (RNA-Seq). The global mRNA interaction profile analysis of TTP, HUR, TIAR, and hnRNP K exhibited valuable information about the post-transcriptional control of inflammation related gene expression with a broad impact on intracellular signaling and temporal cytokine expression.

Regulation of Interleukin-10 Expression

Interleukin (IL)-10 is an essential anti-inflammatory cytokine that plays important roles as a negative regulator of immune responses to microbial antigens. Loss of IL-10 results in the spontaneous development of inflammatory bowel disease as a consequence of an excessive immune response to the gut microbiota. IL-10 also functions to prevent excessive inflammation during the course of infection. IL-10 can be produced in response to pro-inflammatory signals by virtually all immune cells, including T cells, B cells, macrophages, and dendritic cells. Given its function in maintaining the delicate balance between effective immunity and tissue protection, it is evident that IL-10 expression is highly dynamic and needs to be tightly regulated. The transcriptional regulation of IL-10 production in myeloid cells and T cells is the topic of this review. Drivers of IL-10 expression as well as their downstream signaling pathways and transcription factors will be discussed. We will examine in more detail how various signals in CD4⁺ T cells converge on common transcriptional circuits, which fine-tune IL-10 expression in a context-dependent manner.

Neuroprotective Effect of Protein Phosphatase 2A/Tristetraprolin Following Subarachnoid Hemorrhage in Rats

Early brain injury (EBI) following subarachnoid hemorrhage (SAH) can lead to inflammation and neuronal dysfunction. There is a need for effective strategies to mitigate these effects and improve the outcome of patients who experience SAH. The mRNA-destabilizing protein tristetraprolin (TTP) is an anti-inflammatory factor that induces the decay of cytokine transcripts and has been implicated in diseases such as glioma. However, the mechanism of action of TTP in EBI after SAH is unclear. The present study investigated the effects of TTP regulation via phosphorylation in a rat model of SAH by protein phosphatase (PP)2A, which is a pleiotropic enzyme complex with multiple substrate phospho-proteins. We hypothesized that inhibitory phosphorylation of TTP by PP2A would reduce neuroinflammation and apoptosis. To evaluate the function of each factor, the PP2A agonist FTY720, short interfering (si)RNAs targeting TTP and PP2A were administered to rats by intracerebroventricular injection 24 h before SAH. Rats were evaluated with SAH grade, neurological score, brain water content and by western blotting, and terminal deoxynucleotidyltransferase dUTP nick-end labeling. We found that endogenous PP2A and TTP levels were increased after SAH. FTY720 induced PP2A activation would lead to dephosphorylation and activation of TTP and decreased production of tumor necrosis factor (TNF)-α, interleukin (IL)-6, and IL-8. SiRNA-mediated TTP knockdown abolished anti-inflammatory effects of FTY720 treatment, indicating that PP2A was associated with TTP activation in vivo. Decreased TNF-α, IL-6, and IL-8 levels were associated with improvement of neurological function, reduction of brain edema, suppression of caspase-3, and up-regulation of B cell lymphoma-2. These results demonstrated that PP2A activation could enhance the anti-inflammatory and anti-apoptotic effects of TTP, by which it might shed light on the development of an effective therapeutic strategy against EBI following SAH.

Immune-regulation and -functions of eicosanoid lipid mediators

Bioactive lipids regulate most physiological processes, from digestion to blood flow and from hemostasis to labor. Lipid mediators are also involved in multiple pathologies including cancer, autoimmunity or asthma. The pathological roles of lipid mediators are based on their intricate involvement in the immune system, which comprises source and target cells of these mediators. Based on their biosynthetic origin, bioactive lipids can be grouped into different classes [e.g. sphingolipids, formed from sphingosine or eicosanoids, formed from arachidonic acid (AA)]. Owing to the complexity of different mediator classes and the prominent immunological roles of eicosanoids, this review will focus solely on the immune-regulation of eicosanoids. Eicosanoids do not only control key immune responses (e.g. chemotaxis, antigen presentation, phagocytosis), but they are also subject to reciprocal control by the immune system. Particularly, key immunoregulatory cytokines such as IL-4 and IFN-γ shape the cellular eicosanoid profile, thus providing efficient feedback regulation between cytokine and eicosanoid networks. For the purpose of this review, I will first provide a short overview of the most important immunological functions of eicosanoids with a focus on prostaglandins (PGs) and leukotrienes (LTs). Second, I will summarize the current knowledge on immunological factors that regulate eicosanoid production during infection and inflammation.

Assessment of ATP8B1 Deficiency in Pediatric Patients With Cholestasis Using Peripheral Blood Monocyte-Derived Macrophages

Progressive familial intrahepatic cholestasis type 1 (PFIC1), a rare inherited recessive disease resulting from a genetic deficiency in ATP8B1, progresses to liver failure. Because of the difficulty of discriminating PFIC1 from other subtypes of PFIC based on its clinical and histological features and genome sequencing, an alternative method for diagnosing PFIC1 is desirable. Herein, we analyzed human peripheral blood monocyte-derived macrophages (HMDM) and found predominant expression of ATP8B1 in interleukin-10 (IL-10)-induced M2c, a subset of alternatively activated macrophages. SiRNA-mediated depletion of ATP8B1 in IL-10-treated HMDM markedly suppressed the expression of M2c-related surface markers and increased the side scatter (SSC) of M2c, likely via impairment of the IL-10/STAT3 signal transduction pathway. These phenotypic features were confirmed in IL-10-treated HMDM from four PFIC1 patients with disease-causing mutations in both alleles, but not in those from four patients with other subtypes of PFIC. This method identified three PFIC1 patients in a group of PFIC patients undiagnosed by genome sequencing, an identical diagnostic outcome to that achieved by analysis of liver specimens and in vitro mutagenesis studies. In conclusion, ATP8B1 deficiency caused incomplete polarization of HMDM into M2c. Phenotypic analysis of M2c helps to identify PFIC1 patients with no apparent disease-causing mutations in ATP8B1.

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ATP8B1, a causal gene of PFIC1, was expressed in IL-10-induced M2c, a subset of alternatively activated macrophages.

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ATP8B1 deficiency caused incomplete polarization of HMDM into M2c, likely via impairment of IL-10/STAT3 signaling.

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Phenotypic analysis of M2c helps to discriminate PFIC1 from other pediatric liver diseases undiagnosed by genomic analysis.

PFIC1, a rare inherited recessive disease resulting from a genetic deficiency in ATP8B1, progresses to liver failure. PFIC1 shares many clinical and histological features with other subtypes of PFIC, but differs in its therapeutic options. Because genome sequencing in patients with a clinical diagnosis of PFIC cannot always identify disease-causing mutations, an alternative method for diagnosing PFIC1 is desirable. We identified expression of ATP8B1 in IL-10-induced M2c, a subset of macrophages, and demonstrated its contribution to normal phenotypic expression of M2c. The phenotypic analysis of M2c helps to discriminate PFIC1 from other pediatric liver diseases undiagnosed by genomic analysis.

The RNA-binding protein tristetraprolin schedules apoptosis of pathogen-engaged neutrophils during bacterial infection

Protective responses against pathogens require a rapid mobilization of resting neutrophils and the timely removal of activated ones. Neutrophils are exceptionally short-lived leukocytes, yet it remains unclear whether the lifespan of pathogen-engaged neutrophils is regulated differently from that in the circulating steady-state pool. Here, we have found that under homeostatic conditions, the mRNA-destabilizing protein tristetraprolin (TTP) regulates apoptosis and the numbers of activated infiltrating murine neutrophils but not neutrophil cellularity. Activated TTP-deficient neutrophils exhibited decreased apoptosis and enhanced accumulation at the infection site. In the context of myeloid-specific deletion of Ttp, the potentiation of neutrophil deployment protected mice against lethal soft tissue infection with Streptococcus pyogenes and prevented bacterial dissemination. Neutrophil transcriptome analysis revealed that decreased apoptosis of TTP-deficient neutrophils was specifically associated with elevated expression of myeloid cell leukemia 1 (Mcl1) but not other antiapoptotic B cell leukemia/lymphoma 2 (Bcl2) family members. Higher Mcl1 expression resulted from stabilization of Mcl1 mRNA in the absence of TTP. The low apoptosis rate of infiltrating TTP-deficient neutrophils was comparable to that of transgenic Mcl1-overexpressing neutrophils. Our study demonstrates that posttranscriptional gene regulation by TTP schedules the termination of the antimicrobial engagement of neutrophils. The balancing role of TTP comes at the cost of an increased risk of bacterial infections.

The RNA-binding protein Tristetraprolin (TTP) is a critical negative regulator of the NLRP3 inflammasome

The NLRP3 inflammasome is a central regulator of inflammation in many common diseases, including atherosclerosis and type 2 diabetes, driving the production of pro-inflammatory mediators such as IL-1β and IL-18. Due to its function as an inflammatory gatekeeper, expression and activation of NLRP3 need to be tightly regulated. In this study, we highlight novel post-transcriptional mechanisms that can modulate NLRP3 expression. We have identified the RNA-binding protein Tristetraprolin (TTP) as a negative regulator of NLRP3 in human macrophages. TTP targets AU-rich elements in the NLRP3 3'-untranslated region (UTR) and represses NLRP3 expression. Knocking down TTP in primary macrophages leads to an increased induction of NLRP3 by LPS, which is also accompanied by increased Caspase-1 and IL-1β cleavage upon NLRP3, but not AIM2 or NLRC4 inflammasome activation. Furthermore, we found that human NLRP3 can be alternatively polyadenylated, producing a short 3'-UTR isoform that excludes regulatory elements, including the TTP- and miRNA-223-binding sites. Because TTP also represses IL-1β expression, it is a dual inhibitor of the IL-1β system, regulating expression of the cytokine and the upstream controller NLRP3.

Regulation of Postoperative Ileus by Lentivirus-Mediated HuR RNA Interference via the p38/MK2 Signaling Pathway

OBJECTIVES

The objective of the present study is to explore the effect of lentivirus-mediated HuR interference on the development and progression of postoperative ileus and the role of HuR in the regulation of the p38/MAPK-activated protein kinase-2 (MK2) signaling pathway during postoperative ileus.

METHODS

To establish a mouse model of lentiviral transduction, we first determined the optimum effective titer of lentiviral vectors for transduction of the murine small intestine via the abdominal cavity by using hematoxylin and eosin (HE) staining, immunohistochemistry, detection of GFP messenger RNA (mRNA) and protein, and Western blotting. To investigate the effect of HuR interference on gene expression during postoperative ileus, we established a mouse model of postoperative ileus and used RT-PCR to measure the expression of proinflammatory genes, ELISA to measure the expression of serum inflammatory cytokines, immunohistochemistry to evaluate inflammatory cell infiltration in the small intestine, HE staining of paraffin sections to examine the pathology of the small intestine, and Western blotting to measure HuR expression and identify its role in the regulation of the p38/MK2 inflammatory pathway.

RESULTS

We successfully designed a mouse model of intraperitoneal transduction of HuR-RNAi lentivirus. When HuR gene expression was suppressed in a mouse model of postoperative ileus, the infiltration of inflammatory cells, the expression of proinflammatory genes, and the levels of serum inflammatory cytokines were significantly reduced. This reduction in inflammation correlated with reduced cytoplasmic localization of HuR and reduced activation of MK2.

CONCLUSIONS

Within the p38/MK2 signal transduction pathway, HuR may increase the mRNA stability of various inflammatory cytokines, thereby promoting inflammation that causes postoperative ileus. Suppressing the expression of HuR in a postoperative ileus model can effectively suppress the postoperative ileus inflammatory reaction. HuR might serve as a candidate drug target for the prevention and mitigation of postoperative ileus.

Post-transcriptional regulation of cytokine and growth factor signaling in cancer

Cytokines and growth factors regulate cell proliferation, differentiation, migration and apoptosis, and play important roles in coordinating growth signal responses during development. The expression of cytokine genes and the signals transmitted through cytokine receptors are tightly regulated at several levels, including transcriptional and post-transcriptional levels. A majority of cytokine mRNAs, including growth factor transcripts, contain AU-rich elements (AREs) in their 3' untranslated regions that control gene expression by regulating mRNA degradation and changing translational rates. In addition, numerous proteins involved in transmitting signals downstream of cytokine receptors are regulated at the level of mRNA degradation by GU-rich elements (GREs) found in their 3' untranslated regions. Abnormal stabilization and overexpression of ARE or GRE-containing transcripts had been observed in many malignancies, which is a consequence of the malfunction of RNA-binding proteins. In this review, we briefly summarize the role of AREs and GREs in regulating mRNA turnover to coordinate cytokine and growth factor expression, and we describe how dysregulation of mRNA degradation mechanisms contributes to the development and progression of cancer.

STAT3 signaling in immunity

The transcriptional regulator STAT3 has key roles in vertebrate development and mature tissue function including control of inflammation and immunity. Mutations in human STAT3 associate with diseases such as immunodeficiency, autoimmunity and cancer. Strikingly, however, either hyperactivation or inactivation of STAT3 results in human disease, indicating tightly regulated STAT3 function is central to health. Here, we attempt to summarize information on the numerous and distinct biological actions of STAT3, and highlight recent discoveries, with a specific focus on STAT3 function in the immune and hematopoietic systems. Our goal is to spur investigation on mechanisms by which aberrant STAT3 function drives human disease and novel approaches that might be used to modulate disease outcome.

Differences in Tfh Cell Response Between the Graft and Spleen With Chronic Allograft Nephropathy

The aim of this study was to investigate follicular helper T (Tfh) cell response and its difference between renal graft and spleen in a rat renal transplantation model undergoing chronic allograft nephropathy (CAN). Orthotopical kidney transplantations were performed on Fischer (F344) rats and transplanted to Lewis rats, using syngeneic Lewis-Lewis grafts as controls. Tissue samples were collected at 8 weeks posttransplantation. The status of Tfh cell response was assessed by measuring the levels of transcription factor B-cell lymphoma 6 (Bcl-6), interleukin-21 (IL-21), chemokine receptor type 5 (CXCR5), and B-cell-activating factor belonging to the TNF family (BAFF). Tfh cell response was upregulated in both renal graft and spleen of the CAN group compared to the control group. However, Tfh cell response of the spleen was weaker than that of the graft, which was possibly related to the upregulation of splenic Treg activation. Also, the difference between two tissues was partially associated with the different expressions of tristetraprolin (TTP)/IL-10. Our data help improve our understanding of the role of Tfh cell response in the body with CAN and may provide a valuable clue for better treatment of CAN.

Resolution of Inflammation: What Controls Its Onset?

An effective resolution program may be able to prevent the progression from non-resolving acute inflammation to persistent chronic inflammation. It has now become evident that coordinated resolution programs initiate shortly after inflammatory responses begin. In this context, several mechanisms provide the fine-tuning of inflammation and create a favorable environment for the resolution phase to take place and for homeostasis to return. In this review, we focus on the events required for an effective transition from the proinflammatory phase to the onset and establishment of resolution. We suggest that several mediators that promote the inflammatory phase of inflammation can simultaneously initiate a program for active resolution. Indeed, several events enact a decrease in the local chemokine concentration, a reduction which is essential to inhibit further infiltration of neutrophils into the tissue. Interestingly, although neutrophils are cells that characteristically participate in the active phase of inflammation, they also contribute to the onset of resolution. Further understanding of the molecular mechanisms that initiate resolution may be instrumental to develop pro-resolution strategies to treat complex chronic inflammatory diseases, in humans. The efforts to develop strategies based on resolution of inflammation have shaped a new area of pharmacology referred to as "resolution pharmacology."

Tristetraprolin binding site atlas in the macrophage transcriptome reveals a switch for inflammation resolution

Precise regulation of mRNA decay is fundamental for robust yet not exaggerated inflammatory responses to pathogens. However, a global model integrating regulation and functional consequences of inflammation‐associated mRNA decay remains to be established. Using time‐resolved high‐resolution RNA binding analysis of the mRNA‐destabilizing protein tristetraprolin (TTP), an inflammation‐limiting factor, we qualitatively and quantitatively characterize TTP binding positions in the transcriptome of immunostimulated macrophages. We identify pervasive destabilizing and non‐destabilizing TTP binding, including a robust intronic binding, showing that TTP binding is not sufficient for mRNA destabilization. A low degree of flanking RNA structuredness distinguishes occupied from silent binding motifs. By functionally relating TTP binding sites to mRNA stability and levels, we identify a TTP‐controlled switch for the transition from inflammatory into the resolution phase of the macrophage immune response. Mapping of binding positions of the mRNA‐stabilizing protein HuR reveals little target and functional overlap with TTP, implying a limited co‐regulation of inflammatory mRNA decay by these proteins. Our study establishes a functionally annotated and navigable transcriptome‐wide atlas (http://ttp-atlas.univie.ac.at) of cis‐acting elements controlling mRNA decay in inflammation.

Post-transcriptional Regulation of Immunological Responses through Riboclustering

Immunological programing of immune cells varies in response to changing environmental signals. This process is facilitated by modifiers that regulate the translational fate of mRNAs encoding various immune mediators, including cytokines and chemokines, which in turn determine the rapid activation, tolerance, and plasticity of the immune system. RNA-binding proteins (RBPs) recruited by the specific sequence elements in mRNA transcripts are one such modifiers. These RBPs form RBP-RNA complexes known as "riboclusters." These riboclusters serve as RNA sorting machinery, where depending upon the composition of the ribocluster, translation, degradation, or storage of mRNA is controlled. Recent findings suggest that this regulation of mRNA homeostasis is critical for controlling the immune response. Here, we present the current knowledge of the ribocluster-mediated post-transcriptional regulation of immune mediators and highlight recent findings regarding their implications for the pathogenesis of acute or chronic inflammatory diseases.

Resolution of Inflammation: What Controls Its Onset?

An effective resolution program may be able to prevent the progression from non-resolving acute inflammation to persistent chronic inflammation. It has now become evident that coordinated resolution programs initiate shortly after inflammatory responses begin. In this context, several mechanisms provide the fine-tuning of inflammation and create a favorable environment for the resolution phase to take place and for homeostasis to return. In this review, we focus on the events required for an effective transition from the proinflammatory phase to the onset and establishment of resolution. We suggest that several mediators that promote the inflammatory phase of inflammation can simultaneously initiate a program for active resolution. Indeed, several events enact a decrease in the local chemokine concentration, a reduction which is essential to inhibit further infiltration of neutrophils into the tissue. Interestingly, although neutrophils are cells that characteristically participate in the active phase of inflammation, they also contribute to the onset of resolution. Further understanding of the molecular mechanisms that initiate resolution may be instrumental to develop pro-resolution strategies to treat complex chronic inflammatory diseases, in humans. The efforts to develop strategies based on resolution of inflammation have shaped a new area of pharmacology referred to as “resolution pharmacology.”

Impacts of Salmonella enterica Serovar Typhimurium and Its speG Gene on the Transcriptomes of In Vitro M Cells and Caco-2 Cells

Microfold or membranous (M) cells are specialized intestinal epithelial cells responsible for host immunity. The speG mutant of Salmonella Typhimurium (S. Typhimurium) is a nonreplicating strain within human cells to be a candidate vaccine vector for interacting with M cells. We conducted this study to identify the genes are differently expressed between in vitro M cells and Caco-2 cells, and to determine whether S. Typhimurium and speG affect the transcriptomes of both cell types. In vitro M cells and Caco-2 cells were infected with wild-type (WT) S. Typhimurium, its ΔspeG mutant, or none for 1 h for RNA microarrays; the transcriptomes among the 6 pools were pairwisely compared. Genetic loci encoding scaffold (e.g., HSCHR7_CTG4_4, HSCHR9_CTG9_35), long noncoding RNA, membrane-associated protein (PITPNB), neuron-related proteins (OR8D1, OR10G9, and NTNG2), and transporter proteins (MICU2 and SLC28A1) were significantly upregulated in uninfected M cells compared with uninfected Caco-2 cells; and their encoding proteins are promising M-cell markers. Significantly upregulated HSCHR7_CTG4_4 of uninfected in vitro M cells were speG-independently downregulated by S. Typhimurium infection that is a remarkable change representing an important but unreported characteristic of M cells. The immune responses of in vitro M cells and Caco-2 cells can differ and reply on speG or not, with speG-dependent regulation of KYL4, SCTR, IL6, TNF, and CELF4 in Caco-2 cells, JUN, KLF6, and KCTD11 in M cells, or speG-independent modulation of ZFP36 in both cells. This study facilitates understanding of the immune responses of in vitro M cells after administering the S. Typhimurium ΔspeG mutant as a future vaccine vector.

TGF-β decreases the stability of IL-18-induced IFN-γ mRNA through the expression of TGF-β-induced tristetraprolin in KG-1 cells

We have previously reported that transforming growth factor-β (TGF-β) down-regulates interferon-γ (IFN-γ) production in an interleukin-18 (IL-18) treated mouse natural killer (NK) cell line, LNK5E6. In LNK5E6 cells, TGF-β exhibited no inhibition of the IL-18-induced transcription of IFN-γ, but did stimulate the degradation of IFN-γ mRNA induced by IL-18. In the present study, we investigated the mechanism of the down-regulatory effects of TGF-β on IFN-γ mRNA expression in a human myelomonocytic cell line, KG-1, which produces IFN-γ in response to IL-18 alone. Interestingly, IL-18 induced the production of the IFN-γ through the stabilization of IFN-γ mRNA, but not the enhanced transcription of IFN-γ gene. The stability of IFN-γ mRNA was regulated by mRNA destabilizing elements in the 3'untranslated region (UTR) of IFN-γ mRNA, especially adenylate-uridylate (AU)-rich elements (AREs) in the 5' half of 3'UTR. Tristetraprolin (TTP), one of the ARE-binding proteins, destabilizes IFN-γ mRNA, and IL-18 repressed the expression of TTP mRNA. Moreover, TGF-β repressed the IL-18-induced expression of IFN-γ mRNA through the induction of TTP mRNA to destabilize IFN-γ mRNA. Our data is the first to reveal that the crosstalk between IL-18 and TGF-β through the expression of TTP regulates the production of IFN-γ.

Posttranscriptional regulation of cytokine expression

Expression of cytokines and chemokines is regulated at multiple steps during the transfer of the genetic information from DNA sequence to the functional protein. The multilayered control of cytokine expression reflects the need of the immune system to precisely and rapidly adjust the magnitude and duration of immune responses to external cues. Common features of the regulation of cytokine expression are temporal and highly dynamic changes in cytokine mRNA stability. Failures in the timing and extent of mRNA decay can result in disease. Recent advances in transcriptome-wide approaches began to shed light into the complex network of cis-acting sequence elements and trans-acting factors controlling mRNA stability. These approaches led to the discovery of novel unexpected paradigms but they also revealed new questions. This review will discuss the control of cytokine mRNA stability both in the context of high content approaches as well as focused mechanistic studies and animal models. The article highlights the need for systems biology approaches as important means to understand how cytokine mRNA decay helps maintain the immune and tissue homeostasis, and to explore options for therapeutical exploitation of mRNA stability regulation.

Tristetraprolin Limits Inflammatory Cytokine Production in Tumor-Associated Macrophages in an mRNA Decay-Independent Manner

Tristetraprolin (TTP) is an inducible zinc finger AU-rich RNA-binding protein essential for enforcing degradation of mRNAs encoding inflammatory chemokines and cytokines. Most studies on TTP center on the connection between mRNA half-life and inflammatory output, because loss of TTP amplifies inflammation by increasing the stability of AU-rich mRNAs. Here, we focused on how TTP controls cytokine and chemokine production in the nonresolving inflammation of cancer using tissue-specific approaches. In contrast with model in vitro macrophage systems, we found constitutive TTP expression in late-stage tumor-associated macrophages (TAM). However, TTP's effects on AU-rich mRNA stability were negligible and limited by constitutive p38α MAPK activity, which was the main driver of proinflammatory cytokine production in TAMs at the posttranscriptional level. Instead, elimination of TTP caused excessive protein production of inflammatory mediators, suggesting TTP-dependent translational suppression of AU-rich mRNAs. Manipulation of the p38α-TTP axis in macrophages has significant effects on the growth of tumors and therefore represents a means to manipulate inflammation in the tumor microenvironment.

Suppression of antigen presentation by IL-10

Regulated antigen presentation to immune cells determines the effectiveness of an immune response. IL-10 is an immunosuppressive cytokine that regulates immune responses by inhibiting the ability of APCs to present antigens to T cells in a variety of ways. The mechanisms of IL-10-mediated immunosuppression include interference in TLR-mediated or IFNγ-mediated dendritic cell (DC) and macrophage activation as well as direct induction of genes that suppress APC function. In this review we will discuss current studies exploring the molecular mechanisms by which IL-10 suppresses APC function.

RNase L attenuates mitogen-stimulated gene expression via transcriptional and post-transcriptional mechanisms to limit the proliferative response

The cellular response to mitogens is tightly regulated via transcriptional and post-transcriptional mechanisms to rapidly induce genes that promote proliferation and efficiently attenuate their expression to prevent malignant growth. RNase L is an endoribonuclease that mediates diverse antiproliferative activities, and tristetraprolin (TTP) is a mitogen-induced RNA-binding protein that directs the decay of proliferation-stimulatory mRNAs. In light of their roles as endogenous proliferative constraints, we examined the mechanisms and functional interactions of RNase L and TTP to attenuate a mitogenic response. Mitogen stimulation of RNase L-deficient cells significantly increased TTP transcription and the induction of other mitogen-induced mRNAs. This regulation corresponded with elevated expression of serum-response factor (SRF), a master regulator of mitogen-induced transcription. RNase L destabilized the SRF transcript and formed a complex with SRF mRNA in cells providing a mechanism by which RNase L down-regulates SRF-induced genes. TTP and RNase L proteins interacted in cells suggesting that RNase L is directed to cleave TTP-bound RNAs as a mechanism of substrate specificity. Consistent with their concerted function in RNA turnover, the absence of either RNase L or TTP stabilized SRF mRNA, and a subset of established TTP targets was also regulated by RNase L. RNase L deficiency enhanced mitogen-induced proliferation demonstrating its functional role in limiting the mitogenic response. Our findings support a model of feedback regulation in which RNase L and TTP target SRF mRNA and SRF-induced transcripts. Accordingly, meta-analysis revealed an enrichment of RNase L and TTP targets among SRF-regulated genes suggesting that the RNase L/TTP axis represents a viable target to inhibit SRF-driven proliferation in neoplastic diseases.

Post-transcriptional coordination of immunological responses by RNA-binding proteins

Immunological reactions are propelled by ever-changing signals that alter the translational ability of the RNA in the cells involved. Such alterations are considered to be consequential modifications in the transcriptomic decoding of the genetic blueprint. The identification of RNA-binding protein (RBP) assemblies engaged in the coordinative regulation of state-specific RNAs indicates alternative and exclusive means for determining the activation, plasticity and tolerance of cells of the immune system. Here we review current knowledge about RBP-regulated post-transcriptional events involved in the reactivity of cells of the immune system and the importance of their alteration during chronic inflammatory pathology and autoimmunity.

Polyubiquitinated tristetraprolin protects from TNF-induced, caspase-mediated apoptosis

Binding of TNF to its receptor (TNFR1) elicits the spatiotemporal assembly of two signaling complexes that coordinate the balance between cell survival and cell death. We have shown previously that, following TNF treatment, the mRNA decay protein tristetraprolin (TTP) is Lys-63-polyubiquitinated by TNF receptor-associated factor 2 (TRAF2), suggesting a regulatory role in TNFR signaling. Here we demonstrate that TTP interacts with TNFR1 in a TRAF2-dependent manner, thereby initiating the MEKK1/MKK4-dependent activation of JNK activities. This regulatory function toward JNK activation but not NF-κB activation depends on lysine 105 of TTP, which we identified as the corresponding TRAF2 ubiquitination site. Disabling TTP polyubiquitination results in enhanced TNF-induced apoptosis in cervical cancer cells. Together, we uncover a novel aspect of TNFR1 signaling where TTP, in alliance with TRAF2, acts as a balancer of JNK-mediated cell survival versus death.

HIV-1 gp120 activates the STAT3/interleukin-6 axis in primary human monocyte-derived dendritic cells

Dendritic cells (DCs) are fundamental for the initiation of immune responses and are important players in AIDS immunopathogenesis. The modulation of DC functional activities represents a strategic mechanism for HIV-1 to evade immune surveillance. Impairment of DC function may result from bystander effects of HIV-1 envelope proteins independently of direct HIV-1 infection. In this study, we report that exposure of immature monocyte-derived DCs (MDDCs) to HIV-1 R5 gp120 resulted in the CCR5-dependent production of interleukin-6 (IL-6) via mitogen-activated protein kinase (MAPK)/NF-κB pathways. IL-6 in turn activated STAT3 by an autocrine loop. Concomitantly, gp120 promoted an early activation of STAT3 that further contributed to IL-6 induction. This activation paralleled a concomitant upregulation of the STAT3 inhibitor PIAS3. Notably, STAT3/IL-6 pathway activation was not affected by the CCR5-specific ligand CCL4. These results identify STAT3 as a key signaling intermediate activated by gp120 in MDDCs and highlight the existence of a virus-induced dysregulation of the IL-6/STAT3 axis. HIV-1 gp120 signaling through STAT3 may provide an explanation for the impairment of DC function observed upon HIV exposure.

IMPORTANCE

This study provides new evidence for the molecular mechanisms and signaling pathways triggered by HIV-1 gp120 in human DCs in the absence of productive infection, emphasizing a role of aberrant signaling in early virus-host interaction, contributing to viral pathogenesis. We identified STAT3 as a key component in the gp120-mediated signaling cascade involving MAPK and NF-κB components and ultimately leading to IL-6 secretion. STAT3 now is recognized as a key regulator of DC functions. Thus, the identification of this transcription factor as a signaling molecule mediating some of gp120's biological effects unveils a new mechanism by which HIV-1 may deregulate DC functions and contribute to AIDS pathogenesis.

Infection-induced IL-10 and JAK-STAT: A review of the molecular circuitry controlling immune hyperactivity in response to pathogenic microbes

Generation of effective immune responses against pathogenic microbes depends on a fine balance between pro- and anti-inflammatory responses. Interleukin-10 (IL-10) is essential in regulating this balance and has garnered renewed interest recently as a modulator of the response to infection at the JAK-STAT signaling axis of host responses. Here, we examine how IL-10 functions as the “master regulator” of immune responses through JAK-STAT, and provide a perspective from recent insights on bacterial, protozoan, and viral infection model systems. Pattern recognition and subsequent molecular events that drive activation of IL-10-associated JAK-STAT circuitry are reviewed and the implications for microbial pathogenesis are discussed.

The regulation of IL-10 expression

Interleukin (IL)-10 is an important immunoregulatory cytokine and an understanding of how IL-10 expression is controlled is critical in the design of immune intervention strategies. IL-10 is produced by almost all cell types within the innate (including macrophages, monocytes, dendritic cells (DCs), mast cells, neutrophils, eosinophils and natural killer cells) and adaptive (including CD4(+) T cells, CD8(+) T cells and B cells) immune systems. The mechanisms of IL-10 regulation operate at several stages including chromatin remodelling at the Il10 locus, transcriptional regulation of Il10 expression and post-transcriptional regulation of Il10 mRNA. In addition, whereas some aspects of Il10 gene regulation are conserved between different immune cell types, several are cell type- or stimulus-specific. Here, we outline the complexity of IL-10 production by discussing what is known about its regulation in macrophages, monocytes, DCs and CD4(+) T helper cells.

The role of macrophages and dendritic cells in the initiation of inflammation in IBD

In the healthy gastrointestinal tract, homeostasis is an active process that requires a careful balance of host responses to the enteric luminal contents. Intestinal macrophages and dendritic cells (DCs) comprise a unique group of tissue immune cells that are ideally situated at the interface of the host and the enteric luminal environment to appropriately respond to microbes and ingested stimuli. However, intrinsic defects in macrophage and DC function contribute to the pathogenesis of inflammatory bowel diseases, as highlighted by recent genome-wide association studies. Gastrointestinal macrophages and DCs participate in inflammatory bowel disease development through inappropriate responses to enteric microbial stimuli, inefficient clearance of microbes from host tissues, and impaired transition from appropriate proinflammatory responses to anti-inflammatory responses that promote resolution. By understanding how intestinal macrophages and DCs initiate chronic inflammation, new pathogenesis-based therapeutic strategies to treat human inflammatory bowel diseases will be elucidated.

Differential post-transcriptional regulation of IL-10 by TLR2 and TLR4-activated macrophages

The activation of TLRs by microbial molecules triggers intracellular-signaling cascades and the expression of cytokines such as IL-10. Il10 expression is tightly controlled to ensure effective immune responses, while preventing pathology. Maximal TLR-induction of Il10 transcription in macrophages requires signaling through the MAPKs, ERK, and p38. Signals via p38 downstream of TLR4 activation also regulate IL-10 at the post-transcriptional level, but whether this mechanism operates downstream of other TLRs is not clear. We compared the regulation of IL-10 production in TLR2 and TLR4-stimulated BM-derived macrophages and found different stability profiles for the Il10 mRNA. TLR2 signals promoted a rapid induction and degradation of Il10 mRNA, whereas TLR4 signals protected Il10 mRNA from rapid degradation, due to the activation of Toll/IL-1 receptor domain-containing adaptor inducing IFN-β (TRIF) and enhanced p38 signaling. This differential post-transcriptional mechanism contributes to a stronger induction of IL-10 secretion via TLR4. Our study provides a molecular mechanism for the differential IL-10 production by TLR2- or TLR4-stimulated BMMs, showing that p38-induced stability is not common to all TLR-signaling pathways. This mechanism is also observed upon bacterial activation of TLR2 or TLR4 in BMMs, contributing to IL-10 modulation in these cells in an infection setting.